Communities Similarities And Ecological Features Of Zooplanktons In Different Areas Of The Yangtze River Estuary

To compare the zooplankton communities and ecological features in the South Branch(SB), North Branch(NB), North Channel(NC) and offshore(OS) of the Yangtze River estuary, two cruises were carried out in the autumn of 2009 and the spring of 2010. A total of 27 species of zooplankton were identified in the spring and 71 species were collected in autumn. Either in the spring or autumn, the species number and diversity index of the inner area was lower than that of the offshore. In spring, the diversity index of the North Channel was the lowest among four regions, while in autumn, that of the South Branch was the lowest. In spring, Sinocalanus sinensis was the most dominant species of the South Branch, North Branch and North Channel. While in autumn, the dominant species were different for each area.In the meantime, the similarities of zooplankton communities in these regions were analyzed. By hierarchical cluster analysis and non-metric multidimensional scaling(n MDS) method, the results indicated that the zooplankton in Yangtze River estuary can be divided into four communities in spring. Assemblages I in offshore area has a significant difference with the other communities. Affected by ocean masses, the zooplankton in assemblages I was dominant by offshore species, e.g Calanus sinicus with an abundance of 35.30 ind./m3.In the meantime, some estuarine species and coastal species also appeared. With the effect of the Yangtze River runoff, the assemblages II-1(NC), II-2(NB) and II-3(SB) were all dominant by estuarine species. The most important dominant species were the same in these communities(Sinocalanus sinensis), while the abundances were quite different. In autumn, the zooplankton of the Yangtze River was obviously divided in four communities, III, IV, V, and VI. Because of the stronger runoff and the ocean mass, the differences among dominant masses of different regions were larger, which lead to a more obvious difference among communities. Estuarine species prevailed in assemblages III(NC).With a higher salinity, the assemblages IV(NB) was mainly composed of coastal species. Affected by Kuroshio and ocean mass, the assemblages V(OS) was dominant by offshore and marine species. Thanks to the stronger runoff of the Yangtze River, freshwater species dominate the assemblages VI(SB).It was concluded that salinity and water masses were the key factors determining the differences of zooplankton communities in the Yangtze River Estuary, and the differences between the offshore area and the estuarine regions were larger than that among the estuarine regions. To sum up, zooplankton was more abundant in the polyhaline water. Diversity index was largely affected by both species number and abundance percentage. The zooplankton assemblages was mainly influenced by the Yangtze diluted water and offshore water masses. The Kuroshio flowed northward carrying polyhaline water, and brought subtropical species to the offshore of the Yangtze estuary in autumn. Generally speaking, salinity is the most important factor causing the difference of zooplankton assemblages among different regions in the Yangtze estuary. Besides, water masses and estuarine turbidity also influence the zooplankton assemblages.